The substances employed hitherto for this purpose all have certain disadvantages, for example inadequate stability to the action of heat, light or electric fields, or unfavorable elastic and/or dielectric properties.
The compounds excepted by the proviso are compounds of the formula I without lateral fluorination in which m is 0. These bicyclic tolans which are disclosed, for example, in JP 040 216 40, have relatively low solubility and are therefore less suitable for practical applications.
Compounds of the formula I in which m is 0 are disclosed, for example, in JP 063 296 31, but this specification only mentions tolans which have alkyl or alkoxy terminal groups. Compounds of this type exhibit a relatively low optical anisotropy value and a relatively low clearing point and are likewise excepted by the proviso.
Although fluorinated compounds of the formula I containing terminal alkenyl radicals or polar radicals in which m is 0 are amongst the compounds covered by the very broad generic claims in EP 0 648 723 and JP 093 019 01, these documents make absolutely no mention of the advantageous specific lateral fluorination of the compounds of the formula I. EP 543 244, JP 080 601 58, JP 072 343 9 and JP 080 536 74 describe tricyclic fluorinated tolan derivatives, but these likewise differ from the compounds of the formula I according to the invention through the position of the lateral fluorination.